
10
PRELIMINARY
PCIset Overview
A
Memory Gap Range (MC and PB)
The Memory Gap Range can start on any 1 Mbyte boundary, above 1 Mbyte, and can 1, 2, 4, 8, 16, or 32
Mbytes. This region defines a “hole” in system DRAM space where accesses can be directed to the PCI bus.
The Memory Gap Range is used by ISA devices such as LAN or linear frame buffers which are mapped into
the ISA Extended region, or by any EISA or PCI device. The Memory Gap Range must reside above the Low
Memory Gap Range and below the High Memory Gap Range, if it is enabled.
High Memory Gap Range (MC and PB)
The High Memory Space Gap can start on any 1 Mbyte boundary in either the ISA Extended Memory region,
EISA Extended Memory Region, or the Extended Memory Region above 4Gbyte, and can extend up to 64
Gigabytes. It is defined by specifying a start and end address, both on 1 Mbyte boundaries. The High Memory
Gap Range is provided as additional support for memory mapped I/O. The High Memory Gap Range must
reside at the highest address of the three memory gap range registers, if it is enabled.
Figure 4. Expanded View of Extended Memory (ISA)
4.3
Extended Memory (EISA)
The EISA Extended Memory region covers the 16 Mbyte to 4 Gbyte range (1000000h–FFFFFFFFh). This
region is divided into three sections—System BIOS, APIC configuration space, and system memory. The APIC
configuration space is contained within the system memory region (Figure 5). The Low Memory Gap, Memory
Gap, and High Memory Gap ranges can also be enabled in this region.
15MB
System Memory
(Memory or PCI)
10_0000
FF_FFFF
Compatibility
Area
Extended
Memory
(ISA)
Extended
Memory
(EISA)
FFFF_FFFF
100_0000
0
F_FFFF
FF_FFFF
10_0000
Extended
Memory
(above 4GB)
F_FFFF_FFFF
PB Only
MC Only
Note:
If the PCI Frame Buffer Range is enabled in the PB (accesses forwarded to PCI), the Low Memory
Gap in the MC can be used to create a gap in main memory for the corresponding memory address
range (MC ignores these accesses).
High Memory
Gap Range
Memory
Gap Range
PCI Frame
Buffer Range
Low Memory
Gap Range